The application of fluid power is limited only by the ingenuity of the designer, the production engineer or the plant engineer. If the application pertains to lifting, pushing, pulling, clamping, tilting, forcing, pressing or any other straight line (and many rotary) motions, it is probable that fluid power will meet the requirements. The important part that fluid power plays in all phases of industry today is beyond calculation. To indicate its increasing importance, it only needs to be said that new uses are being found and adapted for air and hydraulic systems every day. The present invention was developed in recognition of the importance of fluid power actuators in industry.
The various designs of hydraulic or pneumatic fluid actuators cover such a wide range that it would be impracticable to describe all of them. Hydraulic cylinders recieve fluid pressure from a source of hydraulic fluid and transmit it into lineal force. These cylinders are often referred to as jacks or rams. Pneumatic cylinders use air under pressure. In either case, basic elements of a non-rotating cylinder are: a cylinder tube; a piston; a piston rod; covers or cylinder heads; and packing or seals. The specific application dictates pretty much the design and material of the actuator.
Cushioning, i.e., the gradual deceleration of the piston near to the end of its stroke, is a desirable feature for many applications. It is especially helpful when the piston rod is connected to a heavy load and the piston is travelling at a high rate of speed. It reduces the shock that would otherwise be caused if the piston were allowed to make sharp contact with the cylinder head without any buffer action.
A cushion is a chamber of relatively small diameter into which the cushion nose collar or spud enters as the piston near the end of its stroke so that fluid is trapped in the cylinder tube between the piston and the cylinder head. This fluid is bled off slowly, thereby reducing the rate of piston travel. Two parts of the cylinder are involved in cushioning: the cylinder head and the piston. For purposes of generality, a fluid actuator incorporating a cushioning feature will now be described.
In the operation of such devices, fluid under pressure enters one side of the cylinder portion of the actuator and forces the piston in the opposite direction. Fluid from the other side of the piston is free to flow out of the other end of the cylinder. This action continues until a raised or tapered portion extending from the piston reaches a cavity or opening at the end of the cylinder. The close tolerance between spud or raised portion of the piston and the complementary cavity reduces the flow of fluid out of that end of the cylinder. In effect, it serves as a metering orifice. The reduced volume rate of flow offers resistance to the moving piston and thus cushions the end of the piston stroke.
Very often in the operation of such devices and in the operation of associated machinery components, it is necessary to determine that the piston within the actuator has completed its stroke. This is because the completion of the stroke is used to interlock, trigger or set into motion other process system components.
Most commonly, external mechanical limit switches or knife switches are used to determine the position of a hydraulic piston and to initiate a successive machine cycle. The device disclosed by Allinquant et al. (U.S. Pat. Nos. 4,163,970 and 4,089,512) is typical. These switches are generally bulky and, by virtue of their mechanical nature are often difficult to position relative to the piston rod or cylinder. Because of the large forces associated with hydraulic actuators, externally mounted position detectors are very susceptible to coming out of alignment. In addition, hydraulic oil or dirt frequently "shorts out" the electrical contacts causing the switch to fail. Pneumatic actuators while not subjected to relatively large forces, are cycled at a relatively high rate. Vibration generally leads to misalignment of externally mounted limit switches. Failure of those switches, especially when used as part of a process system interlock, can lead to component failure in other parts of the cycle.
Most processes require fairly tight control; should any of the components come out of calibration or be actuated at the wrong time, the entire process system can come to a complete halt. Chemicals and other raw products are wasted and money is lost. For example, in the manufacture of soap, hydraulic oil leaking out of an actuator through the position detector can contaminate process system chemicals. A position detector that is not susceptible to falling out of calibration as a result of vibration or shock and one that is immune to hostile environmental conditions will be readily accepted by the industry. Heretofore, no one has successfully developed a non-mechanical position detector that can withstand high pressure over a long period of time without leaking.